There may exist a multi-RAT user equipment capable of accessing two or more radio access technologies (RATs). In order to access a specific RAT, a connection to the specific RAT is established based on a request of a user equipment and then data transmission and reception can be performed. Yet, although the multi-RAT UE is capable of accessing two or more RATs, the multi-RAT UE is unable to access multiple RATs at the same time. In particular, although a user equipment is equipped with a multi-RAT capability, the user equipment is unable to simultaneously transmit and receive data via RATs different from each other.
Until now, a concrete procedure and methods enabling a multi-RAT user equipment to simultaneously transmit and receive data via RATs different from each other are not studied yet. In order for the multi-RAT user equipment to transmit and receive data via the RATs different from each other at the same time, it is necessary to have the concrete procedure and the methods.